Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method of releasing a link, performed by a user equipment (UE) in a wireless communication system, and the UE using the method.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D22) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation.
The D2D operation is also referred to as a Proximity Service (ProSe) operation in a sense that a signal is transmitted/received between adjacent devices, and may have various advantages. For example, D2D UE may perform data communication with a high transfer rate and a low delay. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
The D2D UE may also operate as a UE which plays a role of a relay for connecting a sidelink and a cellular link. That is, the D2D UE may operate as a relay UE. The relay UE may play a role of a relay between a specific UE and a network. In this case, the specific UE may be referred to as a remote UE.
Meanwhile, after the relay UE and the remote UE establish a connection, it may be necessary to release the connection. For example, there is a case where a quality of service provided by the relay UE becomes worse or the relay UE needs to preferentially provide a relay service for a different UE. As such, if there is a need to release a link between the relay UE and the remote UE, it may be necessary to specify which method will be used to release the link.